1. Field of the Invention
The present invention relates to a method for manufacturing a single mode optical fiber having a low PMD (Polarization Mode Dispersion), and more particularly to a method for lowering PMD of an optical fiber.
2. Description of the Related Art
Theoretically, a single mode optical fiber with a circular symmetric structure has two orthogonal polarization modes. Generally, an electric field of light propagating through an optical fiber may be considered as an overlap of such two specific polarization modes. In fact, in a real single mode optical fiber, two polarization modes are offset due to incomplete factors such as asymmetric lateral stress or asymmetry of optical fiber geometry. These two modes are propagated with different phase velocities, and thus two polarization modes are propagated with different propagation constants (β1 and β2). This difference of propagation constants is called a birefringence (Δβ), and the increase of birefringence means an increase of the difference of velocities of two polarization modes.
Here, a differential time delay between two polarization modes is called a polarization mode dispersion (PMD), which spreads pulses of light transmitted through an optical fiber, thereby increasing a bit-error rate. Thus, in data transmission through an optical fiber, PMD acts as an essential factor for limiting a capacity of data, so it is not preferable in a signal transmission system, particularly in an optical fiber operating over a long distance.
In order to ensure transmitting/receiving performance of an optical fiber, it is essential to attenuate PMD and thus minimize dispersion of signals. A most preferred scheme to lower PMD is to make an optical fiber with a uniform circular section without mechanical stress or ovality. But, it is substantially difficult to completely remove mechanical stress and ovality of an optical fiber.
For reducing PMD of an optical fiber, it is required to cause a torsional deformation on an optical fiber drawn from an outlet of a furnace. That is to say, a spin is impressed on the optical fiber such that the optical fiber is twisted. As a result, when an optical pulse is transmitted to the optical fiber, the optical pulse is propagated through a slow birefringence axis and a fast birefringence axis, alternately, thereby compensating a relative delay and reducing pulse spreading. It is equivalent to that a local effective refractive index for the pulse becomes equal to an average refractive index of both axes, and this average is applied over the entire pulse length of the optical fiber.
PMD of an optical fiber may be reduced by a spin impressed during the drawing process, and there is proposed a method for optimizing spin conditions in the drawing process. Namely, U.S. Pat. No. 5,298,047 and US 2005/0172675 disclose a method for generating a torsion on an optical fiber to have a spatial frequency (spins/m) with irregular spins impressed on the optical fiber, by canting a guide roller contacted with a coated optical fiber at a certain angle with respect to a drawing axis or linearly reciprocating the guide roller in a direction perpendicular to the drawing axis.
The above conventional method will be explained in more detail with reference to FIGS. 1 to 3.
First, as shown in FIG. 1, an optical fiber preform 1 is melted at a sufficiently high temperature in a furnace 10, and the optical fiber preform 1 is drawn into a single-strand optical fiber 2 from its neck-down portion. Then, the optical fiber passes through a diameter monitor 11, a coating device 12, a curing device 14 and a coating diameter monitor 15. After that, as the melted optical fiber passes over a guide roller 16, a spin is impressed on the optical fiber near the neck-down region by the guide roller, and then the spin-impressed optical fiber is directed to a capstan. At this time, the guide roller 16 cants with being inclined at a certain angle with respect to the drawing axis as shown in FIG. 2, or the guide roller 16′ linearly reciprocates in a direction perpendicular to the drawing axis as shown in FIG. 3, thereby impressing a spin on the optical fiber. At this time, the spin function is a substantial sine function or an amplitude-modulation or frequency-modulation sine function.
In the above spin impressing method for reducing PMD, U.S. Pat. No. 5,298,047 discloses that a maximum spatial frequency of spin impressed on an optical fiber is in excess of 4 spins/m, preferably in excess of 10 turns/m or even 20 spins/m.
However, if the maximum spatial frequency of spin exceeds 4 turns/m, PMD may be reduced, but mechanical stresses caused by shaking of the optical fiber may result in fluctuation of a fiber outer diameter or generation of bubbles during a coating process in the coating device. In addition, even in the case that a maximum spatial frequency of spin is less than 4 turns/m, the optical fiber can have a low PMD due to its symmetric geometry.
Meanwhile, US 2005/0172675 discloses that a maximum spatial frequency of spin impressed on an optical fiber satisfies the following equation. At this time, in the following equation, x is a clad ovality in unit of %, and y is a maximum spatial frequency of spin in unit of turn/m.Exp(24x−12)≦y≦4
However, in the above equation, a minimum value of the maximum spatial frequency of spin for lowering PMD is obtained by considering just a factor about the clad ovality. That is to say, other variables deciding the degree of optical asymmetry of an optical fiber preform, such as a core ovality and eccentricity (ECC), are not considered.
Furthermore, an eccentricity, which is a center deviation caused in the drawing process between the clad and the core, and a core ovality, which is substantially over 5 to 6 times a clad ovality, are not considered at all in the prior art, though they are important factors to determine spin conditions, so it is not easy to obtain a desired PMD reduction effect.